Oxidant activity of tris(2,4,6-trichloro-3,5-dinitrophenyl)methyl radical with catechol and pyrogallol. Mechanistic considerations

Article abstract of DOI:10.1021/jo802559x

The reducing activity of simple polyphenols (PhOH), catechol and pyrogallol, is tested in different solvents in front of tris(2,4,6-trichloro-3, 5-dinitrophenyl)methyl (HNTTM) radical, a stable organic free radical of the TTM series. HNTTM radical is very

Full text of DOI:10.1021/jo802559x

Oxidant Activity of Tris(2,4,6-trichloro-3,5-dinitrophenyl)methyl
Radical with Catechol and Pyrogallol. Mechanistic Considerations
Anna Carreras,† Isaac Esparbe´,‡ Enric Brillas,‡ Jordi Rius,§ Josep Llu´ıs Torres,† and
Luis Julia`*,†
Departament de Qu´ımica Biolo`gica i Modelitzacio´ Molecular, Institut de Qu´ımica AVanc¸ada de Catalunya
(CSIC), Jordi Girona 18-26, 08034 Barcelona, Spain, Departament de Qu´ımica F´ısica, UniVersitat de
Barcelona, Mart´ı Franque`s 1-11, 08028 Barcelona, Spain, abd Institut de Cie`ncia de Materials (CSIC),
Campus de la U.A.B., 08193 Bellaterra, Spain
ljbmoh@cid.csic.es
ReceiVed NoVember 18, 2008
The reducing activity of simple polyphenols (PhOH), catechol and pyrogallol, is tested in different solvents
in front of tris(2,4,6-trichloro-3,5-dinitrophenyl)methyl (HNTTM) radical, a stable organic free radical
of the TTM series. HNTTM radical is very active in electron-transfer reactions to give a very stable
anion. The standard potential for the reduction of HNTTM radical by cyclic voltammetry in different
solvents is E° ) 0.60 ( 5 V vs SCE. In hydroxylic solvents, the electron transfer is a very rapid process
and the electron-donating species is the ionized PhO-, whereas in nonpolar solvents, it is suggested that
the electron transfer is facilitated by the formation of an intermediate complex between HNTTM and
PhOH.
Introduction
hydrogen atoms of the antioxidants, and the ability of these
compounds to release hydrogen atoms depends mainly on the
stability of the generated radicals.3 This is a free-radical reaction
in which a hydrogen atom is transferred from the hydroxyl
group, in the case of polyphenols as antioxidants, to the nitrogen
of DPPH (reaction 1). Sometimes, the number of reduced DPPH
molecules per polyphenol molecule corresponds to the number
of available hydroxyl groups. However, for the majority of
polyphenols tested, the mechanism is more complex.4
Polyphenols show a remarkable ability to be involved in many
oxidation-reduction reactions in chemistry and biology. Many
of them are constituents of natural antioxidant extracts whose
significant role is associated with the ability to scavenge reactive
oxygen species. Among the different methods designed to
measure the activity of the antioxidants,1 stable radicals leading
to color changes upon reaction with them have received great
attention. The hydrazyl radical DPPH is a commercially
available stable radical that has been widely used to test the
antioxidant properties of natural and synthetic polyphenols.1,2
In nonpolar and aprotic solvents, it is extensively accepted that
the activity of DPPH is evaluated by its capacity to abstract
ROH + DPPH f RO• + DPPH-H
(1)
More recently, a new mechanism associated with a proton-
coupled electron transfer (PCET) process has been proposed
for the reduction of free radicals with phenols.5,6 This mecha-
† Institut de Qu´ımica Avanc¸ada de Catalunya.
‡ Universitat de Barcelona.
§ Institut de Cie`ncia de Materials.
(1) Huang, D.; Ou, B.; Prior, R. L. J. Agric. Food Chem. 2005, 53, 1841–
1856, and references therein.
(2) Blois, M. S. Nature 1958, 181, 1199–1200. Brand-Williams, W.; Cuvelier,
M. E.; Berset, C. Lebensm.-Wiss.-Technol. 1995, 28, 25. Sanchez-Moreno, C.;
Larrauri, J. A.; Saura-Calixto, F J. Sci. Food Agric. 1998, 76, 270–276. Goupy,
P.; Hugues, M.; Boivin, P.; Amiot, M. J. J. Sci. Food Agric. 1999, 79, 1625–
1634.
(3) A clear example of the hydrogen atom donating ability of antioxidants
in front of alkyl, alkoxyl, and peroxyl radicals is: Lucarini, M.; Pedrielli, P.;
Pedulli, G. F.; Valginigli, L.; Gigmes, D.; Tordo, P. J. Am. Chem. Soc. 1999,
121, 11546–11553.
(4) Dangles, O.; Fargeix, G.; Dufour, C. J. Chem. Soc., Perkin Trans. 2 1999,
1387–1395. (a) Dangles, O.; Fargeix, G.; Dufour, C. J. Chem. Soc., Perkin Trans.
2 2000, 1653–1663.
2368 J. Org. Chem. 2009, 74, 2368–2373
10.1021/jo802559x CCC: $40.75 2009 American Chemical Society
Published on Web 02/20/2009

Oxidant ActiVity of HNTTM
nism involves an intermediate hydrogen-bonding complex either
between the phenol and the radical (an oxygen-centered radical)
(reaction 2) or between the phenol and the solvent (a hydrogen
bond accepting solvent) (reaction 3) that gives phenoxyl radical
by electron-transfer reaction followed by or concerted with
proton transfer.
respectively. In fact, many of them behave as electrochemical
amphoteric species, being reduced in the cathode and oxidized
in the anode in quasireversible processes. These characteristic
physicochemical properties of the free radicals of the TTM and
PTM series provide a supporting base to use them to test the
antioxidant power of polyphenols by electron-transfer reactions.
In this context, we have reported the synthesis of tris(2,4,6-
trichloro-3,5-dinitrophenyl)methyl (HNTTM) radical with strong
electron-acceptor properties (standard potential E° ) 0.55 V
vs NaCl-saturated calomel electrode (SCE) in CHCl3/MeOH (2:
1), by cyclic voltammetry) and the study of the reducing activity
of some flavan-3-ols with HNTTM.10 More recently, we have
reported the synthesis of a new radical of the PTM series,
tris(2,3,5,6-tetrachloro-4-nitrophenyl)methyl (TNPTM) radical
(E° ) 0.20 V vs SCE in CHCl3/MeOH (2:1), by cyclic
voltammetry) as a new sensor to distinguish selectively the
antioxidant power of catechol (1,2-dihydroxybenzene) and
pyrogallol (1,2,3-trihydroxybenzene). This stable radical is able
to accept an electron from pyrogallol and not from catechol.11
The strong oxidant power of HNTTM radical presumably
involves a reduced species of a great stability. Now we report
the chemical generation and characterization of this species, the
corresponding triphenylcarbanion HNTTM-, isolated as the
tetrabutylammonium salt, and the oxidant activity of HNTTM
radical in front of two simple polyphenols, catechol and
pyrogallol, as essential components of the molecular structure
of many natural antioxidants such as flavonoids. From the results
obtained in these last experiments, we suggest some mechanistic
considerations about the species responsible of the electron
transfers involved in these reactions.
However, polyphenols are partially ionized losing a proton
to give the corresponding anions in ionizable solvents, i.e.,
solvents able to accept protons such as methanol. In these cases,
it has been suggested that the reaction of polyphenols with
radicals consists of a fast electron-transfer process from the
phenoxide anion to radical (reaction 4).7
+
+Y•
PhOH + MeOH h MeOH2 + PhO- f
+
MeOH2 + PhO• + Y-
+
MeOH2 + Y- f MeOH + HY (4)
The studies to measure the antioxidant activity of polyphenols
using DPPH as a persistent free-radical sensor do not provide
clear information of the mechanism involved, and both mech-
anisms, hydrogen abstraction and electron transfer, have been
proposed as alternative reduction paths. We have been engaged
for a long time with stable organic radicals of the tris(2,4,6-
trichlorophenyl)methyl (TTM)8 and perchlorotriphenylmethyl
(PTM)9 series. These free radicals are very persistent species,
both in solid or in solution, due to the presence of the very
bulky polychlorophenyl substituents around the trivalent carbon
atom. As a general rule, they cannot abstract hydrogen atoms
from H-donating compounds because the reactant species have
a great steric hindrance to yield effective molecular collisions.
However, they are very active species in electron-transfer
processes reacting with electron-donating and/or electron-
accepting substrates to give stable anions and/or cations,
(5) Mayer, J. M.; Hrovat, D. A.; Thomas, J. L.; Borden, W. T. J. Am. Chem.
Soc. 2002, 124, 11142–11147. Mayer, J. M. Annu. ReV. Phys. Chem. 2004, 55,
363–390.
Results and Discussion
(6) Galian, R. E.; Litwinienko, G.; Pe´rz-Prieto, J.; Ingold, K. U. J. Am. Chem.
Soc. 2007, 129, 9280–9281. Litwinienko, G.; Ingold, K. U. Acc. Chem. Res.
2007, 40, 222–230.
(7) (a) Litwinienko, G.; Ingold, K. U. J. Org. Chem. 2003, 68, 3433–3438.
(b) Foti, M. C.; Daquino, C.; Geraci, C. J. Org. Chem. 2004, 69, 2309–2314.
(c) Litwinienko, G.; Ingold, K. U. J. Org. Chem. 2004, 69, 5888–5896. (d) Foti,
M. C.; Daquino, C.; Geraci, C. J. Org. Chem. 2004, 69, 2309–2314. (e) Musialik,
M.; Litwinienko, G. Org. Lett. 2005, 7, 4951–4954. (f) Litwinienko, G.; Ingold,
K. U. J. Org. Chem. 2005, 70, 8982–8990. (g) Daquino, C.; Foti, M. C.
Tetrahedron 2006, 62, 1536–1547.
The reaction of HNTTM radical with an aqueous solution of
tetrabutylammonium hydroxide (TBAH) in THF yielded a
strong blue solution of the salt 2.
HNTTM + Bu4NOH
8 Bu4N+•HNTTM-
(5)
THF
(8) Armet, O.; Veciana, J.; Rovira, C.; Riera, J.; Castan˜er, J.; Molins, E.;
Rius, J.; Miravitlles, C.; Olivella, S.; Brichfeus, J. J. Phys. Chem. 1987, 91,
5608–5616. (b) Carilla, J.; Fajar´ı, Ll.; Julia´, L.; Riera, J.; Viadel, Ll. Tetrahedron
Lett. 1994, 35, 6529–6532. (c) Teruel, L.; Viadel, Ll.; Carilla, J.; Fajar´ı, Ll.;
Brillas, E.; San˜e´, J.; Rius, J.; Julia´, L. J. Org. Chem. 1996, 61, 6063–6066. (d)
Carilla, J.; Fajar´ı, Ll.; Julia´, L.; San˜e´, J.; Rius, J. Tetrahedron 1996, 52, 7013–
7024. (e) Gamero, V.; Velasco, D.; Brillas, E.; Julia´, L. Tetrahedron Lett. 2006,
47, 2305–2309.
The one-electron reducing power of the hydroxide ion in polar
solvents other than water, such as DMSO, HMPT, and THF
(10) (a) Torres, J. L.; Varela, B.; Brillas, E.; Julia´, L. Chem. Commun. 2003,
74, 75. (b) Jime´nez, A.; Selga, A.; Torres, J. L.; Julia´, L. Org. Lett. 2004, 6,
4583–4586.
(11) Torres, J. L.; Carreras, A.; Jime´nez, A.; Brillas, E.; Torrelles, X.; Rius,
J.; Julia´, L. J. Org. Chem. 2007, 72, 3750–3756.
(9) Ballester, M.; Riera, J.; Castan˜er, J.; Badia, C.; Monso´, J. M. J. Am. Chem.
Soc. 1971, 93, 2215–2225. (b) Ballester, M. Acc. Chem. Res. 1985, 18, 380–
387, and references cited therein.
J. Org. Chem. Vol. 74, No. 6, 2009 2369

Carreras et al.
TABLE 1. Angles (deg) between Planesa of the Molecular
Structure of HNTTM Radical
A-D
A-B
A-C
A-A′
A-A′′
51(1)
79(1)
80(1)
76.5(10)
67.4(10)
a Planes are defined as follows: A (C1 to C7, N1 N2), B (C2, N1, O2,
O3), C (C6, N2, O3, O4), D (C4, C4′, C4′′, C7), A′ (C1′ to C7′, N1′
N2′) and A′′ (C1′′ to C7′′, N1′′ N2′′).
partially inhibited by the twist angles around the central carbon
atom. HNTTM radical exhibits a quasireversible reduction
process in cyclic voltammetry with a standard potential (E° )
0.58 V vs SCE) in CH2Cl2 solution, attributed to the addition
of one electron to the trivalent carbon atom.10a This positive
and large value, compared with that of the TTM radical (E° )
-0.66 V vs SCE) is a consequence of the strong electron
acceptor properties of the six m-nitro groups in the phenyl rings.
The nitro group has traditionally been considered as a strong
electron-withdrawing group, with a mode of action both
inductive and through resonance. In the particular case of
HNTTM radical, the nitro groups act by inductive effect. In
fact, the E° value for the HNTTM radical is the highest value
found into the radicals of the TTM series.)
The reactivity of simple polyphenols such as catechol (1,2-
dihydroxybenzene) and pyrogallol (1,2,3-trihydroxybenzene)
with stable radical 1• was measured on the bench from
equimolecular solutions of both reactants in solvents of different
polarity such as benzene and CHCl3 and in a polar solvent with
hydroxylic hydrogen atoms such as a mixture of CHCl3/MeOH
(2:1). The experiments with concentrations of the reactants
(∼0.1 M) much higher than those of the analytical tests were
made during short (4 h) and long (47 h) times to assess and
characterize all the HNTTM-containing products of the reaction
in each trial. To measure reaction rates and stoichiometric factors
of these redox processes, obviously any coupling reaction
between HNTTM radical and any radical species derived from
the decomposition of polyphenols must be discarded. These
experiments were carried out as shown in the Supporting
Information, and the reactivity results obtained are collected in
Table 2. All of them were very clean and simple processes
yielding the diamagnetic species 1 as the result of the reduction
of 1•. In some of the tested experiments, the compounds reacted
very slowly or did not react at all, being recovered part of the
species 1•.
FIGURE 1. Perspective view of the anion of the salt 2 with atom
numbering. To indicate the presence of a 3-fold symmetry axis in the
molecule, the symmetry-related atoms of C4 have been labeled as C4′
and C4′′.
has been reported in the literature.12,13 This one-electron
donation character of the hydroxide anion is assisted by the
presence of powerful electron-acceptor substrates. Some per-
chlorinated aromatic substrates, such as perchlorinated triph-
enylmethyl radicals with enhanced electron affinity, have been
very suitable substrates for this kind of electron-transfer
reactions. Salt 2 is very stable in solid and in solution at room
temperature, and it has been fully characterized. The absorption
spectrum in chloroform solution shows a typical and broadband
at λ(ε) ) 499 nm (26700 dm3 mol-1 cm-1). The thermal stability
has been measured by thermogravimetric (TG) and differential
scanning calorimetry (DSC) analysis, and it is stable at
temperatures up to 273 °C. Salt 2 yields small dark violet needles
by slow evaporation of a saturated solution in methanol. The
molecular and crystal structure of 2 was solved from an X-ray
powder diffraction pattern measured on a conventional labora-
tory equipment (Cu KR1,2 radiation) (see the Supporting
Information for experimental, structure solution and refinement,
and Figures S1-S4). The perspective view of the anion of salt
2 with the atom numbering is shown in Figure 1.
Due to the limited resolution of the powder data (minimum
d spacing =1.43 Å), final bond distances are largely influenced
by the target values introduced in the restraints. Consequently,
the most reliable and useful information supplied by the
refinement are the torsion angles and the dihedral angles between
mean planes describing the overall geometry of the molecule
(Table 1 as well as the crystal packing of the compound. Twist
angles of phenyl rings around the plane defined by the sp2 C7
and C4, C4′, C4′′ (51°) are very similar to the corresponding
angles in the molecular structure of TNPTM radical.11 Similarly,
NO2 planes are forced out of the phenyl planes (80°) due to the
presence of o-chlorine atoms.
The stability of the reduced species, carbanions, derived from
radicals of the TTM series is determined by the reversible
reduction potential of the corresponding radical. At the same
time, the reduction potential is predominantly related to the
effects of the substituents in the phenyl rings, although the extent
of localization of the charge density in the phenyl rings is
From an analysis of Table 2, it follows that 1• reacts very
slowly with catechol in benzene and CHCl3 but gently in CHCl3/
MeOH (2:1). On the other hand, 1• reacts smoothly with
pyrogallol in benzene and CHCl3, although the reaction proceeds
very fast in CHCl3/MeOH (2:1). The different results obtained
in these experiments depended both on the electron-donating
strength of the polyphenols and on the nature of the solvent
employed. Thus, the reaction rates of 1•with catechol and with
pyrogallol either in benzene, CHCl3 or in CHCl3/MeOH (2:1)
are in accordance with their different anodic peak potentials in
(12) Pearson, R. G. J. Am. Chem. Soc. 1986, 108, 6109–6114.
(13) Ballester, M. Pure Appl. Chem. 1967, 15, 123–151. Ballester, M.; Riera,
J.; Castan˜er, J.; Badia, C.; Monso, J. M. J. Am. Chem. Soc. 1971, 93, 2215–
2225. Ballester, M.; Castan˜ar, J.; Riera, J.; Iba´n˜ez, A.; Pujadas, J. J. Org. Chem.
1982, 47, 259–264. Ballester, M.; Riera, J.; Castan˜er, J.; Rodr´ıguez, A.; Rovira,
C.; Veciana, J. J. Org. Chem 1982, 47, 4498–4505. Ballester, M.; Castan˜er, J.;
Riera, J.; Pujadas, J. J. Org. Chem. 1984, 49, 2884–2887. Ballester, M.; Castan˜ar,
J.; Riera, J.; Pujadas, J.; Armet, O.; Onrubia, C.; Rio, J. A. J. Org. Chem. 1984,
49, 770–778. Roberts, J. L.; Sugimoto, H., Jr.; Barrette, W. C.; Sawyer, D. T.,
Jr. J. Am. Chem. Soc. 1985, 107, 4556–4557. Ballester, M.; Pascual, I. J. Org.
Chem. 1991, 56, 841–844. Lu, Y.; Liu, J.; Diffee, G.; Liu, D.; Liu, B. Tetrahedron
Lett. 2006, 47, 4597–4599.
2370 J. Org. Chem. Vol. 74, No. 6, 2009

Oxidant ActiVity of HNTTM
TABLE 2. Reactivity of Catechol and Pyrogallol with 1• in
Benzene, CHCl3, and CHCl3/MeOH (2:1) (Equimolecular Solutions)a
reagent
time
(h)
Catechol 1•
(%)b1 (%)
Pyrogallol 1•
(%)b1 (%)
solvent
benzene
4
47
4
47
4
92
71
96
73
34
11
8
29
4
27
66
89
76
51
75
50
<5
0
24
49
25
50
>95
100
CHCl3
CHCl3/MeOH (2:1)
47
FIGURE 2. Absorption spectra of 1• (blue), 1- (purple) (λ ) 494 nm),
and 1• + pyrogallol in CHCl3/MeOH (2:1) after a few minutes of
reaction (black) and a simulation of the black spectrum by a combina-
tion of 1• and 1- (red).
a Initial concentration of polyphenol and HNTTM radical, 0.12 M.
b Recovered radical.
TABLE 3. Cyclic Voltammetry Parameters for the Oxidation of
Catechol and Pyrogallol and for the Reduction of Radical 1•
carried out in CHCl3/MeOH (2:1) and benzene as solvents and
monitored by electronic spectroscopy, recording the decay of
the maximum absorbance of 1• (λmax ) 384 and 387 nm in
benzene and CHCl3/ MeOH (2:1), respectively) as a conse-
quence of the electron addition to give anion 1-. The experi-
ments were effected with a 1•/polyphenol molar ratio of ∼5:1
and an initial concentration of 1• of 54 µM. The reactions were
completed when the absortivities in the electronic spectra
remained practically constants. The n-values of the stoichiometry
of polyphenols were calculated using eq 6
catechol
Ep /V
pyrogallol
Ep /V
radical
a
a
1• E°/Va (Ecp/V)a
a
a
solvent
benzene
CHCl3
CHCl3/MeOH (2:1)
b
1.64
1.27
1.00
-
0.65 (0.48)
0.58 (0.50)
0.55 (0.46)
1.15
0.82
a
a
Anodic peak potential (Ep ), standard potential (E°) and cathodic
a
peak potential (Ep ) for the substrate (10-3 M) with 0.4
M
M
tetrahexylammonium hexafluorophosphate in benzene and 0.1
tetrabutylammonium perchlorate in CHCl3 or CHCl3/MeOH (2:1), on Pt
at scan rates of 50 mV s-1 and 25.0 °C. b Not determined by partial
adsorption of the substrate on Pt.
A0 - Af
cyclic voltammetry14 (Ep [pyrogallol] < Ep [catechol]) (Table
3; Figures S5-S7, Supporting Information). So, the reactivity
of 1•with pyrogallol is greater than with catechol and it follows
the order CHCl3/MeOH (2:1) . CHCl3 ∼ benzene. The
dramatic increase in CHCl3/MeOH (2:1) may be due to the
partial ionization of the phenol to phenolate ion in hydroxylic
solvents, as it has been accepted by other authors.7 In this case,
the anodic peak potential of the phenolate ion is much lower
than that of phenol.11
a
a
n )
(6)
ε·c
where A0 and Af are the initial and final absorbances of the
radical 1•, respectively, cis the initial concentration of the
antioxidant, and ε is the molar absortivity of the radical.
The n values for the reactions of 1• with catechol and
pyrogallol in CHCl3/MeOH (2:1) and 1• with pyrogallol in
benzene are given in Table 4. While catechol reduces two
molecules of radical per molecule, pyrogallol reduces up to three
molecules of radical per molecule. Two steps of the decay of
the absorbance of radical 1•, one fast and one slow, with the
antioxidant, catechol, or pyrogallol, in CHCl3/MeOH and only
one slow decay step with pyrogallol in benzene can be
distinguished in the course of the reducing processes. Calculated
second-order rate constants (k1) for the first fast step of the
reactions in CHCl3/MeOH and for the only slow step in benzene
are shown in Table 4.
All these reactions lead to the stable charged species 1- by
electron transfer from the polyphenol and, finally, to the
diamagnetic triphenylmethane 1 by abstraction of a hydrogen
ion. The presence of the charged species 1- has been confirmed
by electronic spectroscopy. Thus, electronic spectra of the
products present in the course of the reaction between 1• and
pyrogallol in CHCl3/MeOH (2:1) and in benzene are shown in
Figures 2 and 3, respectively. In Figures 2 and 3 (right), the
lowest-energy band intensities for the products resulting after
a few minutes of mixturing of both reactants, 1• and pyrogallol,
are displayed together with a simulation of the spectra of single
species 1• and 1- in the same solvents. The simulated spectra
were obtained by a linear combination of the spectra of 1• and
1- showing that both species are present in solution in ∼19%
and ∼81% in CHCl3/MeOH and in ∼99% and ∼1% in benzene,
respectively. Figure 3 (left) displays the evolution of the
absorption band of a more concentrated solution of 1• and
pyrogallol, clearly observing the presence of the small absorption
band of 1- immersed into the very large band of 1•.
Reactions of catechol and pyrogallol with radical 1• in CHCl3/
MeOH (2:1) and pyrogallol with 1• in benzene could be
evaluated after a moderate time (5.5 h) because they are
practically completed and the number of electrons transferred
per molecule of polyphenol were approximately 3 for pyrogallol
and 2 for catechol. The rate constant values were estimated by
using a simple kinetic model reported by Dangles et al.15 and
are shown in Table 4 (Graphics of the kinetics of the reactions
are displayed in the Supporting Information; see Figures
S8-S10).
Experiments of catechol with radical 1• in benzene were also
distinguished by only one very slow step. The number of
electrons transferred from catechol to 1• after a long reaction
time (13 h) corresponds to approximately one electron per
To determine the kinetic parameters of the reactions of
catechol and pyrogallol with radical 1•, these processes were
(14) Cyclic voltammograms for the oxidation of catechol and pyrogallol
solutions (1 mM) in CHCl3 and CHCl3/MeOH 2:1 (v/v) with tetrabutylammonium
perchlorate (0.1 M) on Pt at 25 °C and in benzene with tetrahexylammonium
hexafluorophosphate (0.3 M) on Pt at 25 °C were recorded. It was not possible
to assign an anodic peak potential for the oxidation of pyrogallol in benzene
due to the strong adsorption of the oxidized species in the electrode.
(15) Goupy, P.; Dufour, C.; Loonis, M.; Dangles, O. J. Agric. Food Chem.
2003, 51, 615–622.
J. Org. Chem. Vol. 74, No. 6, 2009 2371

Carreras et al.
FIGURE 3. Left: Evolution of the lowest energy band intensity of a solution of 1• (120.2 µM) and pyrogallol (1.63 mM) in dry benzene at different
times (first band in black, 1• in benzene; intervals of 10 min). Right: Lowest energy band in the absorption spectra of 1• (black), 1- (green) (λ, 499
nm), theoretical combination: 1• + 1- (red), and 1• (54.0 µM) + pyrogallol (10.6 µM) in dry benzene after 3 min of reaction (blue).
TABLE 4. Observed Rate Constants and Stoichiometric Factors
for the Reactionsa of Radical 1• with Catechol and Pyrogallol in
CHCl3/MeOH (2:1) and with Pyrogallol in Dry Benzene
SCHEME 2
1•:polyphenol
solvent
molar ratiob
k1 (M-1 s-1
)
nc
catechol
CHCl3/MeOH (2:1)
5.1
4.6
5.1
1123 ( 95
5115 ( 82
14.5 ( 0.4
1.9
3.1
3.0
pyrogallol CHCl3/MeOH (2:1)
pyrogallol benzene
b
a Time of reaction, 5.5 h. Initial concentrations of radical 1•, 54 ( 2
µM. c Number of electrons transferred per molecule of polyphenol.
the corresponding 1- (see Figure 3). Now, if we pay attention
to the onset potential values for pyrogallol and radical 1• in
benzene from their corresponding cyclic voltammograms, i.e.,
when the electron transfer on Pt begins to take place, we obtain
SCHEME 1
a
c
Eonset ∼ 1.1 V for the oxidation of the former and Eonset
∼
0.75 V for the reduction of the latter, the electron-transfer
reaction between pyrogallol and 1• should be endoergonic. To
overcome this difficulty, it is reasonable to envisage the
formation of a complex intermediate between both species. The
strength of interactions between the phenol and the radical 1•
can seriously affect the electrochemistry of both reactants
providing an efficient intermolecular electron transfer from the
pyrogallol. This intermediate may be constituted by hydrogen-
bonding between the nitro oxygen atoms of the radical and the
hydroxyl hydrogen atoms of the polyphenol, increasing the
electron density on the phenolic oxygen and making it easier
to be oxidized (Scheme 2).
This sequence of steps explains favorably the detection of
the reduced species 1- by electronic spectroscopy. Thus, once
the electron is transferred into the complex, this rapidly
dissociates leading to the free ions. Anion 1- has been clearly
detected by spectroscopy before the protonation of 1-- takes
place. However, it is not discounted at all that the protonation
of 1- might take place in part into the complex as the acidity
of the phenol significantly increases after one-electron oxidation
to the radical cation.
molecule of catechol, and values of the rate constant in the initial
steps of these processes as low as k1 ) 0.6 M-1 s-1 were
estimated.16
The rather fast reactions of catechol and pyrogallol with
radical 1• in CHCl3/MeOH (2:1) are due, as mentioned above,
to the fact that polyphenols are partially ionized to charged
species PhO- in this medium and these species can be more
a
a
easily oxidized (Ep ) 0.25 V vs SCE for catechol and Ep
)
0.06 V vs SCE for pyrogallol in CHCl3/MeOH (2:1) in the
presence of an excess of tetrabutylammonium hydroxide)11 than
the corresponding molecular species PhOH (see Table 3).
Judging from the anodic peak potentials of catechol and
pyrogallol, and the cathodic peak potential of 1• (see Table 3),
both reactions should be exoergonic. The electron-deficient
radical 1• reacts very fast by electron transfer reaction with the
generally low concentration of PhO- present in equilibrium with
PhOH. Thus, the sequential proton loss-electron transfer
(SPLET) mechanism, first reported by Litwinienko7a and Foti,7b
occurs in these processes as described in Scheme 1.
In conclusion, the reactions between radical 1• and pyrogallol
(PhOH) in different solvents occur by electron-transfer reaction
to the SOMO of 1• to give the charged 1-, which is consecu-
tively protonated to 1H. In hydroxylic solvents, the electron
transfer is a very rapid process and the electron-donating species
is the ionized PhO-, whereas in nonpolar and nonhydroxylic
solvents, the electron transfer is facilitated by assuming an
intermediate complex between 1• and PhOH. An additional
experimental proof against the hydrogen atom abstraction
mechanism of this last process is that TTM radical does not
The reaction of radical 1• with pyrogallol in benzene deserves
especial mention. Benzene is a nonionizable solvent with a very
low dielectric constant and it shows weak interactions with
charged and polar species. However, the reaction of pyrogallol
with radical 1•, although slow as shown by the low value of the
rate constant (Table 4), takes place through the generation of
(16) Values of k1 were estimated proceeding with the Dangles’ kinetic model.
Now, the Af value in eq 8 corresponds to the visible absorbance (λ ) 384 nm)
of the radical 1• after 13 h when the reaction is not finished.
2372 J. Org. Chem. Vol. 74, No. 6, 2009

Oxidant ActiVity of HNTTM
the spectrometer cell, was added a freshly prepared solution (90.8
µM) (390 µL) of the polyphenol in the same solvent. The UV-vis
spectra were recorded at different time intervals.
react at all with pyrogallol in benzene, although TTM and
HNTTM have similar stability due to the steric hindrance
provided by the six chlorine atoms around the trivalent carbon.
The difference between both radicals resides in their electro-
chemical properties, HNTTM radical being much more oxidant
(E° ) 0.58 V vs SCE) than TTM radical (E° ) -0.66 V vs
SCE).11
All of the procedures were carried out with an excess of radical.
The rate constant of the electron transfer from phenol to HNTTM
radical was estimated with a simple and general kinetic model
reported by Dangles et al.15 making no hypothesis about the
mechanism of polyphenol degradation, and only the fast steps of
the reactions in CHCl3/MeOH were subjected to a kinetic analysis.
Hence, in eqs 7 and 8, the digit 3 represents the number of moles
of HNTTM radical that 1 mol of pyrogallol is capable to reduce,
A0 is the initial absorbance of the radical 1• in the UV-visible
spectra, Af is the absorbance at the end of the fast step in the
reactions in CHCl3/MeOH and at the end of the single step in the
reaction in benzene, and c is the initial concentration of pyrogallol.
Experimental Section
Synthesis of Tetrabutylammonium Tris(2,4,6-trichloro-3,5-dini-
trophenyl)methide (2). To a stirred solution of tris(2,4,6-trichloro-
3,5-dinitrophenyl)methane (1) (269 mg; 0.3 mmol) in THF (15 mL)
at rt was added an aqueous solution of tetrabutylammonium
hydroxide (TBAH) (1.5 M) (26 µL; 0.4 mmol), and the mixture
was left in argon for a period of time (6 h). The precipitate (253
mg) was filtrated off, washed with water, and dried under reduced
pressure to give salt 2 (253 mg; 73%): IR (KBr) 2973 (w), 2929
(w), 2871 (w), 1553 (s), 1465 (w), 1358 (m), 1222 (w), 945 (w),
833 (w), 784 (w), 735 (w), 677 (w), 638 (w) cm. Anal. Calcd for
C35H36Cl9N7O12: C, 39.4; H, 3.4; N, 9.2; Cl, 29.9. Found: C, 39.7;
H, 3.6; N, 8.8; Cl, 29.2.
-d[TNPTM]/dt ) k · 3·[pyrogallol][TNPTM]
) k1[pyrogallol][TNPTM]
(7)
(8)
1 - Af/A
k1c
ln
) -
t
1 - Af/A0
A0/Af - 1
Reactions of Radical 1• with Polyphenols. Equimolecular
solutions of tris(2,4,6-trichloro-3,5-dinitrophenyl)methyl (1•) radical
(50 mg) and catechol (6.7 mg) or pyrogallol (7.6 mg) in three
different solvents (50 mL), CHCl3, CHCl3/MeOH (2:1), and dry
C6H6, were stirred (47 h) at rt in the dark and in an inert atmosphere
(Ar). The solutions were treated with water, and the organic
fractions, dried over sodium sulfate, were evaporated off to yield
a solid identified as a mixture of radical 1• and methane 1 by IR
and UV-visible spectra. The course of the reactions was monitored
by UV-vis spectroscopy after 4 and 47 h of reaction.
Acknowledgment. Financial support for this research from
the MCT (Spain) through project AGL2006-12210-C03-02/ALI
is gratefully acknowledged.
Supporting Information Available: Synthesis and X-ray
powder diffraction study (experimental, structure solution, and
refinement) of salt 2; final atomic coordinates and observed and
calculated X-ray diffraction patterns. Graphics for kinetics of
HNTTM with pyrogallol and catechol. Cyclic voltammograms.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Kinetic Measurements. An estimation of the kinetics of
reactions of radical 1• with catechol and pyrogallol in CHCl3/MeOH
(2:1) and dry C6H6 at rt were performed. Procedure: To 3 mL of a
freshly prepared solution (60 µM) of 1• in the solvent, placed in
JO802559X
J. Org. Chem. Vol. 74, No. 6, 2009 2373